Multilayer printed wiring board and method of manufacturing the same

ABSTRACT

This invention provides a multilayer printed wiring board having flat via holes. This is a multilayer printed wiring board formed by alternately laminating multiple metal foils and insulating layers, in which an interlayer connection via pad provided in a first insulating layer, a wiring circuit and an interlayer connection via bottom pad of a second insulating layer are provided in the same surface layer and at least the interlayer connection via pad and the interlayer connection via bottom pad of the second insulating layer have the same thickness.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a multilayer printed wiring board whichhas high connection reliability of interlayer connection vias andpermits high-density wiring and a method of manufacturing thismultilayer printed wiring board.

2. Description of the Related Art

Methods of forming a circuit in a printed wiring board are divided intothe subtractive method which involves forming an etching resist on ametal conductor layer of copper foil etc. and forming a wiring circuitby etching the part of metal conductor layer not covered by this etchingresist; and the additive method which involves forming a plating resistof a pattern reverse to a circuit and forming a wiring circuit byprecipitating a coating on an opening of this plating resist.

Because the manufacturing steps in the subtractive method are easierthan in the additive method, it is possible to manufacture circuits atlow cost. However, in the subtractive method it is necessary to apply anelectroless coating and an electrolytic coating to the whole insulatingsubstrate in forming full through holes and blind via holes and the like(hereinafter referred to as via holes) and, therefore, the thickness ofthe conductor layer to be etched equals to the sum of metal foil plus acoating. Thus, the subtractive method was not very suitable for theformation of a good fine wiring circuit, for example, a wiring circuithaving L/S (line/space) of not more than 75 μm/75 μm.

In contrast, the additive method, which is favorable for the formationof a fine wiring circuit, requires that a wiring circuit be formed byprecipitating a coating on an insulating layer and, therefore, comparedto a case as in the subtractive method where an insulating substrate inwhich metal foil has been laminated on an insulating layer is processed,the additive method had problems such that the adhesion of the wiringcircuit is poor.

Furthermore, in terms of the design of printed wiring circuits, wiringcircuits are formed nonuniformly in the plane of the substrate and,therefore, when wiring circuits are formed by selective coating as inthe additive method, currents are concentrated more than necessary inportions of coarse wiring circuits and variations occur in the thicknessof wiring circuit, thereby posing problems such that it is difficult toensure impedance matching.

As methods of manufacturing printed wiring boards to solve suchproblems, there has already been reported, for example, a technique asdisclosed in JP-A-2004-319994.

That is, the method including forming a wiring circuit in the part ofmetal foil beforehand, forming then at least a barrier layersubstantially on the whole area followed by forming a nonthrough hole,and applying first an electroless coating and then an electrolyticcoating substantially to the whole area including via holes, therebyforming via holes. By this method it is possible to form via holes ofhigh connection reliability at the same time with the formation of afine wiring circuit.

In the above-described conventional method, in particular, materialswith a metal foil thickness of 12 μm, 9 μm and 7 μm are used when theline/space of a fine wiring circuit decreases from 30 μm to 20 μm and 10μm, respectively.

In this method, when a wiring circuit is refined and the thickness ofmetal foil is reduced, an insulating layer is laminated on an upperlayer and the wiring circuit and via holes are connected. When a wiringcircuit is used as a via hole bottom pad of an insulating layer of theupper layer, because of thinness of the metal foil, this posed theproblem that when a nonthrough hole is formed by laser processing forexample, it becomes impossible to form the nonthrough hole which piercesthrough the via hole bottom pad. That is, a depression as indicated by Cin FIG. 4 is formed, making it impossible to maintain interlayerconnection.

The metal foil height of each layer in a case where a via hole issuperposed on a via hole differs from that in a case where a via hole isformed on metal foil. For example, when a via hole is provided on metalfoil, this posed the problem that the interior of the via hole is notsufficiently filled with a metal coating, resulting in the formation ofa recessed portion. That is, a recess as indicated by D in FIG. 5 isformed.

In view of the above-described problems and actual situation, thepresent invention has as its object the provision of a multilayerprinted wiring board provided with flat via holes.

SUMMARY OF THE INVENTION

To achieve the above object, the present invention provides a multilayerprinted wiring board formed by alternately laminating multipleinsulating layers and metal foils, in which an interlayer connection viapad provided in a first insulating layer, a wiring circuit and aninterlayer connection via bottom pad of a second insulating layer areprovided in the same surface layer and at least the interlayerconnection via pad and the interlayer connection via bottom pad of thesecond insulating layer have the same thickness.

Owing to this construction, the depth of nonthrough holes of via holesis the same; and therefore, interlayer connection vias of multipleinsulating layers also become flat.

To achieve the above object, the present invention provides a multilayerprinted wiring board, in which the interlayer connection via pad of thefirst insulating layer and the interlayer connection via bottom pad ofthe second insulating layer are formed from metal foil, a barrier layerand a metal coating.

Owing to this construction, it is possible to form a fine circuit bymetal foil, and the thickness of the interlayer connection via padbecomes equal to the thickness of the interlayer connection via bottomof the insulating layer of the upper layer, resulting in improvedconnection reliability of via holes.

To achieve the above object, the present invention provides a multilayerprinted wiring board, in which the barrier layer is formed from anelectroless coating of the same kind as the metal foil plus anelectrolysis nickel coating, an electroless coating of the same kind asthe metal foil plus an electrolytic silver coating or an electrolesscoating of the same kind as the metal foil plus an electrolytic tincoating.

Owing to this construction, the fine circuit formed from metal foil isprotected by the barrier layer, and variations in wiring circuits do notoccur any more and impedance control becomes easy.

To achieve the above object, the present invention provides a multilayerprinted wiring board, in which the barrier layer is formed from anelectroless nickel coating, an electroless silver coating or anelectroless tin coating.

Owing to this construction, the protection of the fine circuit formedfrom metal foil by the barrier layer is enhanced, variations in wiringcircuits do not occur any more and impedance control becomes easier.

To achieve the above object, the present invention provides a multilayerprinted wiring board, in which the interlayer connection via pad of thefirst insulating layer and the interlayer connection via bottom pad ofthe second insulating layer are formed from metal foil plus anelectroless coating of the same kind as the metal foil plus anelectrolytic coating of the same kind as the metal foil.

Owing to this construction, a plating resist can be used in place of abarrier layer; and therefore, variations in wiring circuits do not occurany more and impedance control becomes easier.

To achieve the above object, the present invention provides a method ofmanufacturing a multilayer printed wiring board, which involves thesteps of: preparing an insulating substrate, the front and back surfacesof which have metal foil, and forming a circuit by laminating a firstinsulating layer and metal foil at least on one surface of theinsulating substrate; forming a nonthrough hole for interlayerconnection; forming a barrier layer substantially on the whole area;forming a metal coating substantially on the whole area; forming anetching resist in an interlayer connection via pad and an interlayerconnection via bottom pad of an interlayer insulating layer of an upperlayer; etching the metal coating thereby to remove an excess metalcoating; exfoliating the etching resist; removing an exposed barrierlayer; forming a second insulating layer and a metal layer; forming acircuit; forming a nonthrough hole for interlayer connection; forming abarrier layer substantially on the whole area; forming a metal coatingsubstantially on the whole area; forming an etching resist in aninterlayer connection via pad for circuit formation in an outermostlayer; removing an excess metal coating; exfoliating the etching resist;and removing the barrier layer.

With this manufacturing method, the depth of nonthrough holes of viaholes is the same; and therefore, interlayer connection vias of multipleinsulating layers also become flat.

To achieve the above object, the present invention provides a method ofmanufacturing a multilayer printed wiring board, in which the barrierlayer formed substantially in the whole area is formed from anelectroless coating of the same kind as the metal foil plus anelectrolytic nickel coating, an electroless coating of the same kind asthe metal foil plus an electrolytic silver coating or an electrolesscoating of the same kind as the metal foil plus an electrolytic tincoating.

With this manufacturing method, the fine circuit formed from metal foilis protected by the barrier layer; and therefore, it becomes easy toform via holes from a fine circuit and a metal coating.

To achieve the above object, the present invention provides a method ofmanufacturing a multilayer printed wiring board, in which the barrierlayer formed substantially in the whole area is formed from anelectroless nickel coating, an electroless silver coating or anelectroless tin coating.

With this manufacturing method, the metal foil forms a barrier layer bya dissimilar metal; and therefore, selective etching becomes possibleand fine circuits can be easily formed by the subtractive method.

To achieve the above object, the present invention provides a method ofmanufacturing a multilayer printed wiring board, in which the nonthroughhole for interlayer connection is formed by a laser.

With this manufacturing method, it is possible to perform hole makingefficiently by leaving a metal coating in the bottom pad of a nonthroughhole.

To achieve the above object, the present invention provides a method ofmanufacturing a multilayer printed wiring board, in which the nonthroughhole for interlayer connection is formed by exposure and development.

With this manufacturing method, even in a case where the number of holesis large, the processing time is short and productivity is improved.

To achieve the above object, the present invention provides a method ofmanufacturing a multilayer printed wiring board, which involves thesteps of: preparing an insulating substrate, the front and back surfacesof which have metal foil and forming a circuit by laminating a firstinsulating layer and metal foil on the insulating substrate; forming anonthrough hole for interlayer connection; forming an electroless metalcoating substantially in the whole area; forming an electrolytic metalcoating substantially in the whole area; forming an etching resist in aninterlayer connection via pad and an interlayer connection via bottompad of an interlayer insulating layer of an upper layer; etching themetal coating thereby to remove an excess metal coating; exfoliating theetching resist; forming a second insulating layer and a metal layer;forming a circuit; forming a nonthrough hole for interlayer connection;forming an electroless metal coating substantially in the whole area;forming an electrolytic metal coating substantially in the whole area;forming an etching resist in an interlayer connection via pad forcircuit formation in an outermost layer; removing an excess metalcoating; and exfoliating the etching resist.

With this manufacturing method, the depth of nonthrough holes of viaholes is the same; and therefore, interlayer connection vias of multipleinsulating layers also become flat.

To achieve the above object, the present invention provides a method ofmanufacturing a multilayer printed wiring board, in which the nonthroughhole for interlayer connection is formed by a laser.

With this manufacturing method, it is possible to perform hole makingefficiently by leaving a metal coating in the bottom pad of a nonthroughhole.

To achieve the above object, the present invention provides a method ofmanufacturing a multilayer printed wiring board, in which the nonthroughhole for interlayer connection is formed by exposure and development.

With this manufacturing method, even in a case where the number of holesis large, the processing time is short and productivity is improved.

According to the present invention, fine circuits can be formed byreducing the conductor thickness to the wiring circuits from metal foilalone by the subtractive method, and by ensuring that the interlayerconnection via hole pad and the interlayer connection via pad bottomhave the same thickness, it is possible to provide a multilayer printedwiring board having flat via holes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional diagram to explain a multilayer printedwiring board in the first embodiment of the present invention;

FIG. 2 is a schematic sectional diagram to explain a multilayer printedwiring board in the second embodiment of the present invention;

FIGS. 3(a) to 3(o) are each a schematic sectional diagram to explain thesteps of a manufacturing method of the present invention;

FIG. 4 is a schematic sectional diagram to explain a conventionalmultilayer printed wiring board; and

FIG. 5 is a schematic sectional diagram to explain another conventionalmultilayer printed wiring board.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a schematic sectional diagram to explain a multilayer printedwiring board in the first embodiment of the present invention and showsan example of a multilayer printed wiring board 1 of a stacked structurehaving two buildup layers. In FIG. 1, a first insulating layer 3 andmetal foil are laminated on a lower-layer wiring circuit 2 to form afirst wiring circuit 4. As the metal foil it is preferable to enumeratecopper foil, aluminum foil, silver foil, tin foil, etc. However, it ispreferable to use copper foil, which is inexpensive and easy to handle.In the first wiring circuit 4 (formed from metal foil alone), a barrierlayer 5 a and a barrier layer 6 a are formed on the top surfaces of apad portion of a first interlayer connection via 5 and a pad portion ofa second interlayer via bottom 6 formed in a second insulating layer 7,respectively, and a metal coating 5 b and a metal coating 6 b arefurther provided, whereby a first insulating layer interlayer via pad Aand a second insulating layer interlayer via bottom pad B arerespectively formed. The first wiring circuit 4, the first insulatinglayer interlayer connection via pad A and the second insulating layerinterlayer via bottom pad B are provided in the same surface layer, andbesides, the first insulating layer interlayer via pad A and the secondinsulating layer interlayer via bottom pad B have the same thickness. Asa result, the formation of a recess of a second interlayer connectionvia 10 can be suppressed and at the same time, even in a case where themetal foil used in the wiring circuit 4 is thin, it is possible to solvethe problem that the interlayer connection bottom is depressed duringthe formation of a nonthrough hole by laser processing, for example.

The second insulating layer 7, along with metal foil, is laminated onthe surface of the first wiring circuit 4, whereby a second wiringcircuit 8 (formed from metal foil alone) and second interlayerconnection vias 9, 10 are formed. The second interlayer connection vias9, 10 are constituted by barrier layers 9 a, 10 a and metal coatings 9b, 10 b, respectively.

The barrier layers 5 a, 6 a and 9 a, 10 a are formed from an electrolesscoating of the same kind as the metal foil plus an electrolytic nickelcoating, an electroless coating of the same kind as the metal foil plusan electrolytic silver coating, an electroless coating of the same kindas the metal foil plus an electrolytic tin coating and the like. Oralternatively, the barrier layers 5 a, 6 a and 9 a, 10 a may be formedfrom an electroless nickel coating, an electroless silver coating, anelectroless tin coating, a plating resist and the like.

Next, FIG. 2 is a schematic sectional diagram to explain a multilayerprinted wiring board in the second embodiment of the present invention.Although basically FIG. 2 is the same as FIG. 1, it shows an example ofa multilayer printed wiring board 1 in which the connection structurebetween the first interlayer connection via hole and the secondinterlayer connection via hole is a staggered via structure.Incidentally, in the stacked structure, the first and second interlayerconnection via holes are formed in directly above and below with respectto each other, whereas in the staggered structure, the first and secondinterlayer connection via holes are formed in a staircase pattern.

Although in FIGS. 1 and 2 above, the descriptions were given along withan example in which the interlayer connection vias 5, 9, 10 are filledwith a metal coating, it is also possible to precipitate a metal coatingonly on an inner surface of a nonthrough hole, whereby a BVH recess isformed. And the BVH recess may be filled with the resin of an insulatinglayer, an electrically conductive paste, etc. Although in FIGS. 1 and 2,the descriptions were given of an example in which two buildup layersare formed on one side of the basic structure, it is needless to saythat the present invention can be carried out in structures of 3 layers,4 layers and more layers or in a both-side buildup structure.

Subsequently, a manufacturing method of the present invention will bedescribed by using FIGS. 3(a) to 3(o).

First, as shown in FIG. 3(a), a first insulating layer 22 and metal foil23 are laminated on a lower-layer wiring circuit 21. Next, as shown inFIG. 3(b), a wiring circuit 24 is formed on the metal foil 23 by thesubtractive method. Next, as shown in FIG. 3(c), a non through hole 25for interlayer connection is formed by laser processing or by theexposure and development of a photosensitive insulating resin andsubjected to desemear treatment. Next, as shown in FIG. 3(d), a barrierlayer 26 is formed substantially in the whole area. The barrier layer 26is formed from an electroless coating of the same kind as the metal foil23 plus an electrolysis nickel coating, an electroless coating of thesame kind as the metal foil 23 plus an electrolytic silver coating or anelectroless metal coating of the same kind as the metal foil 23 plus anelectrolytic tin coating and the like. Or alternatively, the barrierlayer 26 may be formed from an electroless nickel coating, anelectroless silver coating, an electroless tin coating or a platingresist and the like.

Next, as shown in FIG. 3(e), an electrolytic metal coating 27 is appliedsubstantially to the whole area. A generally used copper coating ispreferable as the electrolytic metal coating 27. Next, as shown in FIG.3(f), an etching resist 28 is formed by the photolithography method. Asthe etching resist used here, a dry film, a photosensitive liquid resin,an electrodeposited coating film, etc. can be enumerated.

Next, as shown in FIG. 3(g), after the removal of an excess electrolyticmetal coating 27 by etching, the etching resist 28 is exfoliated. Next,as shown in FIG. 3(h), the exposed barrier layer 26 is removed byetching, whereby it is ensured that the thickness of an interlayerconnection via pad A formed from the wiring circuit 24 (metal foil 23),the barrier layer 26 and the electrolytic metal coating 27 and thethickness of an interlayer connection via bottom pad B similarly formedfrom the wiring circuit 24 (metal foil 23), the barrier layer 26 and theelectrolytic metal coating 27 in a second insulating layer are madeequal to each other. As a result of this, it becomes possible to form asecond interlayer connection via in a flat condition without a recess.Incidentally, unless the thickness of A and B is the same, it becomesnecessary to deposit an excessive metal coating to remove the recess inthe step of an electrolytic metal coating, which consequently poses aproblem that productivity is low.

Next, as shown in FIG. 3(i), a second insulating layer 29 and metal foilare laminated. Next, as shown in FIG. 3(j), a second wiring circuit 30is formed. Next, as shown in FIG. 3(k), a nonthrough hole 25 is formedin the second insulating layer 29 and a barrier layer 26 is formedsubstantially in the whole area. Next, as shown in FIG. 3(l), anelectrolytic metal coating 27 is formed substantially in the whole area.Next, as shown in FIG. 3(m), an etching resist 28 is formed by thephotolithography method. Next, as shown in FIG. 3(n), an excess ofelectrolytic metal coating 27 is removed by etching and the etchingresist 28 is exfoliated. Next, as shown in FIG. 3(o), the exposedbarrier layer 26 is removed by etching, whereby a multilayer printedwiring board 31 of the present invention is completed. Incidentally, ina wiring circuit of an outermost layer, a solder resist to protect thewiring circuit from solder during the mounting of parts is formed.

1. A multilayer printed wiring board formed by alternately laminatingmultiple insulating layers and metal foils, in which an interlayerconnection via pad provided in a first insulating layer, a wiringcircuit and an interlayer connection via bottom pad of a secondinsulating layer are provided in the same surface layer and at least theinterlayer connection via pad and the interlayer connection via bottompad of the second insulating layer have the same thickness.
 2. Themultilayer printed wiring board according to claim 1, wherein theinterlayer connection via pad of the first insulating layer and theinterlayer connection via bottom pad of the second insulating layer areformed from metal foil, a barrier layer and a metal coating.
 3. Themultilayer printed wiring board according to claim 2, wherein thebarrier layer is formed from an electroless coating of the same kind asthe metal foil plus an electrolysis nickel coating, an electrolesscoating of the same kind as the metal foil plus an electrolytic silvercoating or an electroless coating of the same kind as the metal foilplus an electrolytic tin coating.
 4. The multilayer printed wiring boardaccording to claim 2, wherein the barrier layer is formed from anelectroless nickel coating, an electroless silver coating or anelectroless tin coating.
 5. The multilayer printed wiring boardaccording to any one of claims 1 to 3, wherein the interlayer connectionvia pad of the first insulating layer and the interlayer connection viabottom pad of the second insulating layer are formed from metal foilplus an electroless coating of the same kind as the metal foil plus anelectrolytic coating of the same kind as the metal foil.
 6. A method ofmanufacturing a multilayer printed wiring board, comprising the stepsof: preparing an insulating substrate, the front and back surfaces ofwhich have metal foil and forming a circuit by laminating a firstinsulating layer and metal foil at least on one surface of theinsulating substrate; forming a nonthrough hole for interlayerconnection; forming a barrier layer substantially in the whole area;forming a metal coating substantially in the whole area; forming anetching resist in an interlayer connection via pad and an interlayerconnection via bottom pad of an interlayer insulating layer of an upperlayer; etching the metal coating thereby to remove an excess of metalcoating; exfoliating the etching resist; removing an exposed barrierlayer; forming a second insulating layer and a metal layer; forming acircuit; forming a nonthrough hole for interlayer connection; forming abarrier layer substantially in the whole area; forming a metal coatingsubstantially in the whole area; forming an etching resist in aninterlayer connection via pad for circuit formation in an outermostlayer; removing an excess metal coating; exfoliating the etching resist;and removing the barrier layer.
 7. The method of manufacturing amultilayer printed wiring board according to claim 6, wherein thebarrier layer formed substantially in the whole area is formed from anelectroless coating of the same kind as the metal foil plus anelectrolytic nickel coating, an electroless coating of the same kind asthe metal foil plus an electrolytic silver coating or an electrolesscoating of the same kind as the metal foil plus an electrolytic tincoating.
 8. The method of manufacturing a multilayer printed wiringboard according to claim 6, wherein the barrier layer formedsubstantially in the whole area is formed from an electroless nickelcoating, an electroless silver coating or an electroless tin coating. 9.The method of manufacturing a multilayer printed wiring board accordingto any one of claims 6 to 8, wherein the nonthrough hole for interlayerconnection is formed by a laser.
 10. The method of manufacturing amultilayer printed wiring board according to any one of claims 6 to 8,wherein the nonthrough hole for interlayer connection is formed byexposure and development.
 11. A method of manufacturing a multilayerprinted wiring board, comprising the steps of: preparing an insulatingsubstrate, the front and back surface's of which have metal foil andforming a circuit on the insulating substrate; forming a nonthrough holefor interlayer connection; forming an electroless metal coatingsubstantially in the whole area; forming an electrolytic metal coatingsubstantially in the whole area; forming an etching resist in aninterlayer connection via pad and an interlayer connection via bottompad of an interlayer insulating layer of an upper layer; etching themetal coating thereby to remove an excess of metal coating; exfoliatingthe etching resist; further forming an insulating layer and a metallayer; forming a circuit; forming a nonthrough hole for interlayerconnection; forming an electroless metal coating substantially in thewhole area; forming an electrolytic metal coating substantially in thewhole area; forming an etching resist in an interlayer connection viapad for circuit formation in an outermost layer; removing an excess ofmetal coating; and exfoliating the etching resist.
 12. The method ofmanufacturing a multilayer printed wiring board according to claim 11,wherein the nonthrough hole for interlayer connection is formed by alaser.
 13. The method of manufacturing a multilayer printed wiring boardaccording to claim 11, wherein the nonthrough hole for interlayerconnection is formed by exposure and development.